Weak reference objects, which do not prevent their referents from being
made finalizable, finalized, and then reclaimed.

Package java.lang.ref Description

Provides reference-object classes, which support a limited degree of
interaction with the garbage collector. A program may use a reference object
to maintain a reference to some other object in such a way that the latter
object may still be reclaimed by the collector. A program may also arrange to
be notified some time after the collector has determined that the reachability
of a given object has changed.

Package Specification

A reference object encapsulates a reference to some other object so
that the reference itself may be examined and manipulated like any other
object. Three types of reference objects are provided, each weaker than the
last: soft, weak, and phantom. Each type
corresponds to a different level of reachability, as defined below. Soft
references are for implementing memory-sensitive caches, weak references are
for implementing canonicalizing mappings that do not prevent their keys (or
values) from being reclaimed, and phantom references are for scheduling
pre-mortem cleanup actions in a more flexible way than is possible with the
Java finalization mechanism.

Each reference-object type is implemented by a subclass of the abstract
base Reference class. An instance of one of
these subclasses encapsulates a single reference to a particular object, called
the referent. Every reference object provides methods for getting and
clearing the reference. Aside from the clearing operation reference objects
are otherwise immutable, so no set operation is provided. A
program may further subclass these subclasses, adding whatever fields and
methods are required for its purposes, or it may use these subclasses without
change.

Notification

A program may request to be notified of changes in an object's reachability by
registering an appropriate reference object with a reference
queue at the time the reference object is created. Some time after the
garbage collector determines that the reachability of the referent has changed
to the value corresponding to the type of the reference, it will add the
reference to the associated queue. At this point, the reference is considered
to be enqueued. The program may remove references from a queue either
by polling or by blocking until a reference becomes available. Reference
queues are implemented by the ReferenceQueue
class.

The relationship between a registered reference object and its queue is
one-sided. That is, a queue does not keep track of the references that are
registered with it. If a registered reference becomes unreachable itself, then
it will never be enqueued. It is the responsibility of the program using
reference objects to ensure that the objects remain reachable for as long as
the program is interested in their referents.

While some programs will choose to dedicate a thread to removing reference
objects from one or more queues and processing them, this is by no means
necessary. A tactic that often works well is to examine a reference queue in
the course of performing some other fairly-frequent action. For example, a
hashtable that uses weak references to implement weak keys could poll its
reference queue each time the table is accessed. This is how the WeakHashMap class works. Because the ReferenceQueue.poll method simply
checks an internal data structure, this check will add little overhead to the
hashtable access methods.

Automatically-cleared references

Soft and weak references are automatically cleared by the collector before
being added to the queues with which they are registered, if any. Therefore
soft and weak references need not be registered with a queue in order to be
useful, while phantom references do. An object that is reachable via phantom
references will remain so until all such references are cleared or themselves
become unreachable.

Reachability

Going from strongest to weakest, the different levels of reachability reflect
the life cycle of an object. They are operationally defined as follows:

An object is strongly reachable if it can be reached by some
thread without traversing any reference objects. A newly-created object is
strongly reachable by the thread that created it.

An object is softly reachable if it is not strongly reachable but
can be reached by traversing a soft reference.

An object is weakly reachable if it is neither strongly nor
softly reachable but can be reached by traversing a weak reference. When the
weak references to a weakly-reachable object are cleared, the object becomes
eligible for finalization.

An object is phantom reachable if it is neither strongly, softly,
nor weakly reachable, it has been finalized, and some phantom reference refers
to it.

Finally, an object is unreachable, and therefore eligible for
reclamation, when it is not reachable in any of the above ways.